As one of typical aromatic polyesters, polyethylene terephthalate is known. The polyethylene terephthalate (PET) is used as the material of bottles containing beverages and other liquids, so-called PET bottles. Most of spent PET bottles are disposed of by incineration or landfill. However, these disposal methods are not preferable from the viewpoints of reuse of resource and environmental protection. So, it is desired to develop recycling technology for recovering spent PET bottles. Necessity for developing such technology is increased with growing consumption of PET bottles. Several methods have been developed in each of which, PET is decomposed for recovering monomer components therefrom, and thus recovered monomer components can be used for reproduction of new PET bottles.
Among these conventional methods, typical ones are listed below.
(1) Methanolysis method where solvolysis is carried out with liquid methanol; PA1 (2) Glycolysis method where solvolysis is carried out with ethylene glycol; PA1 (3) Ester interchange method where methyl esterification is carried out after the Glycolysis method (2); and PA1 (4) Alkali hydrolysis method where hydrolysis is carried out with alkali solution. PA1 (A) Method for obtaining terephthalic acid where PET is hydrolized with super critical water; and PA1 (B) Method for recovering monomer components where super critical methanol and PET are reacted in a batch type reactor as disclosed in Japanese Patent No. 2807781.
Further, in these days, the following two methods are proposed.
However, the above methods have drawbacks respectively.
In the method (1) (Methanolysis method), its reaction temperature is so low of about 450 K that its reaction rate is small. Accordingly, catalysts are required to accelerate its reaction, which leads to increased cost.
In either of the method (2) (Glycolysis method) and method (3) (Ester interchange method), catalyst is required like the method (1) (Methanolysis method). Further, either of their reaction steps is complicated.
In either of method (4) (Alkali hydrolysis method) and method (A) (Method for obtaining terephthalic acid), it is difficult to purify the terephthalic acid obtained by the hydrolysis. Particularly, in the method (A) (Method for obtaining terephthalic acid), the reaction conditions are very severe. Concretely, the reaction temperature is very high (same as or higher than 450 K) and the reaction pressure is also very high (same as or higher than 30 MPa). Further, it is substantially impossible to recover perfectly the monomer components, since one resultant monomer component (ethylene glycol) is decomposed due to catalytic action, which is performed by another resultant monomer component (terephthalic acid) in the solution.
In the method (B) (Method for recovering monomer components) disclosed in Japanese Patent No. 2807781, the following process is performed. First, methanol and aromatic polyester are fed in the weight proportion of the same as or higher than 10 mol, preferably 20 to 70 mol of methanol per 1 mol of aromatic dicarboxylic acid in the aromatic polyester. Then, they are reacted under the conditions of temperature of 512.6 to 673 K, preferably 523 to 653 K and of pressure of 3 to 30 MPa, preferably 5 to 25 MPa. But this Japanese Patent does not refer to an industrial process for continuously producing monomer components. In the method of this patent, when the reaction products are recovered after cooling of the reactor, the recovered dimethyl terephthalate is precipitated due to its low solubility to the methanol, and therefore, it takes trouble to discharge this product. Additionally, since the dimethyl terephthalate, on recovering, tends to be mixed with not-yet-decomposed PET, a separation step is required to separate the not-yet-decomposed PET and the produced dimethyl terephthalate. Further, because of low level of recovering efficiency, this method is not suitable for practical use in the field of industry.
It is, therefore, the first object of the present invention is to provide a process by which monomer components can be produced continuously from the aromatic polyester with high yield as well as by which the monomer components can be recovered and separated simply under industrially advantageous conditions.
The second object of the present invention is to, in this process, discharge the large amount of reaction products from the reactor during the reaction and to increase the throughout per unit time for improved productivity. By attaining this second object, after reaction is carried out continuously for a certain period and the pressure in the reactor is reduced to the atmospheric pressure, the reaction products can be recovered not in batch operation but in continuous operation.
The third object of the present invention is to increase the purity of monomer components as products and to prevent oligomer components from remaining in the monomer components.